DESCRIPTION (Investigator's Abstract): This research continues the study of the neurological disturbances that follow hemispheric cerebral injury, causing hemiparesis. Our study emphasizes cerebral injury resulting from a stroke. Our research program focuses on three themes. These are: I) AIM I: DETERMINE THE GAIN OF THE STRETCH REFLEX IN SPASTIC MUSCLES. Here we reexamine the issue of altered segmental reflex gains in spastic hypertonia. In recent years a number of investigators have argued that intrinsic motoneuronal excitability is enhanced in such a way that input/output properties of the motoneuron pool are modified. We propose to reevaluate this question by focusing on particular measures of joint impedance and on electromyographic measures which are less impacted by changes in mechanical and electrical properties of muscle in chronic neurological injury. These studies on reflex gains at the whole muscle level will be coupled with studies of single motor unit discharge in spastic muscle, to determine whether there are systematic changes in synaptic processing in spastic motoneurons. 2) AIM II: TO EVALUATE POSSIBLE MECHANISMS OF MUSCULAR WEAKNESS IN BRAIN INJURY: Our findings in the past have been that there are substantial reductions in mean motoneuron firing rates in paretic muscle. These are often, although not uniformly associated with changes in force EMG relationships. We plan to evaluate the natural history of changes in electromyographic activity of paretic muscles and to correlate these changes with the key findings of the neuroradiological studies (primarily CAT scans) which are obtained at the time of admission of stroke subject to our affiliate hospital. 3) AIM III: TO EVALUATE DESCENDING PATHWAYS ACTIONS IN MEDIATING DISTURBANCES OF MUSCULAR COORDINATION IN HEMIPARESIS We plan to continue our studies on the origins of the disturbances in muscle synergic patterns that arise following brain injury, by emphasizing the potential contributions of descending pathways from the brainstem, especially vestibulospinal and reticulospinal systems. These systems are characterized by extensive divergence, and may induce functional coupling of muscles innervated from separate spinal segments. Taken overall, these studies will allow a continued increase in our knowledge of the key factors underlying the motor deficits in stroke, leading potentially to more rational diagnosis and to more effective rehabilitation therapies.